Fix build failures (#1747)

Summary:
Problem
`util::PrepareInputTensors` is deprecated and is unavailable when linking to `executorch.a`.  This is causing build failures.

Solution
Remove the `util::PrepareInputTensors` call and set the method input using a `std::vector` as the data storage.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1747

Reviewed By: guangy10, kirklandsign

Differential Revision: D53228671

Pulled By: shoumikhin

fbshipit-source-id: 373c3a2ac7afb1269f02c8cabf118f13120be7c4